340. Longest Substring with At Most K Distinct Characters

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Given a string s and an integer k, return the length of the longest contiguous substring of s that uses at most k different characters. A character that appears several times in the substring counts once. If k is 0, the answer is 0.

Example

s = "eceba", k = 2  ->  3   ("ece")
s = "aa", k = 1     ->  2

Constraints

• 1 <= s.length <= 5 * 10^4
• 0 <= k <= 50
• s consists of English letters